The present invention relates generally to signal acquisition probes and more particularly to a differential measurement probe having retractable double cushioned variable spacing probing tips with electrical over stress (EOS) and electrostatic discharge (ESD) protection capabilities.
Differential time domain reflectometry (TDR) probes are used to launch step pulses onto devices under test and receive reflected return pulses from the device. The return pulses are coupled to a sampling head which generates discrete samples of the return signal. Due to the variability of the spacing of test points on the device under test, it is desirable to provide a differential TDR probe with variable spacing probing tips. One such differential TDR probe is the A0134332 Differential TDR probe, manufactured and sold by Inter-Continental Microwave, Santa Clara, Calif. The variable spacing differential TDR probe has individual TDR probes that are mounted to a flat spring using two screws. Each TDR probe has a metallic housing with one end of the housing having a threaded connector for connecting a signal cable. A substantially rectangular member extends outward from below the connector and has a threaded aperture for receiving the screw that secures the TDR probe to the flat spring. Below the rectangular member is a circular portion that transitions into a narrow rectangular probe tip member. The probe tip member has an aperture that receives an RF pin and dielectric member. The RF pin is electrically connected to a central signal contact of the treaded connector. Additional apertures are formed in the narrow rectangular probe tip member for receiving ground pogo pins. The various apertures allow the ground pogo pins to be positioned at various distances from the RF pin. A variable spacing adjustment clamp is position over the TDR probes adjacent to the narrow rectangular probe tip members. The adjustment clamp has a “U” shaped portion and a flat portion with the two portions being secured together with screws. The two opposing sides of the “U” shaped member have threaded apertures that receive adjustment cap screws that extend through the sides of the “U” shaped member and into interior space of the “U”. Treaded apertures are formed in the base of the “U” shaped member that intersect the threaded apertures in the opposing sides of the “U” shaped member. Each threaded aperture in the base receives a set screw that is tightened on the adjustment cap screws.
Positioning of the RF pins are accomplished by loosening the set screws on the adjustment cap screws and turning the adjustment cap screws to move each TDR probes closer together or farther apart. The flat spring to which the TDR probes are attached causes outward pressure on the probes to force them against the adjustment cap screws. The screws holding the TDR probes to the flat spring may also be loosened to allow rotational movement of the probes. When the RF tip and the ground pogo pins are positioned correctly, the set screws and the flat spring screws are tightened.
While the above described variable spacing TDR probe allows movement of the RF tips and the ground pogo pins, the design of the probe has drawbacks. For example, the RF pins are fixed in the TDR probes so that RF pin had no axial movement. This requires that the RF pins of the differential probe be positioned in the same lateral plane to make good contact with the device under test. If the RF tips are not in the same lateral plane, one of the RF pins will be subject to more axial force in order for the other RF pin to make contact. If this axial force is excessive, then damage can occur to one or both of the RF pins. This requires the replacement of the RF pin and the dielectric member. Further, this variable spacing differential probe does not have the capability to protect the sampling head from electrostatic voltages that may be present on the device under test. This will result in damage to the sampling circuit in the sampling head.
Ultra high speed sampling heads used in time domain reflectometry typically dictate extremely low capacitances. This introduces unique problems. Sampling devices are much more sensitive to static discharge residing on a device under test. The small geometry of the sampling diodes in the sampling head often dictate low breakdown voltages. The low parasitic capacitance at the sampling head input means that for a given device under test (DUT) static discharge, there will be a higher transient voltage at the sampler input because of the reduced charge sharing effect. It is therefore important to neutralize any static charge on the device under test before the sampling head input is coupled to the device under test.
Another type of variable spacing differential measurement probe is described in U.S. Pat. No. 6,704,670. The variable spacing measurement probe has first and second typically cylindrical probe barrel. Each probe barrel is constructed of an electrically conductive material that extends partially outside of a probe unit housing. A probe barrel nose cone is attached to each of the exposed probe barrels. Each probe nose cone is generally conical in form and made of an insulating material. The longitudinal axis of each probe barrel nose cone extends from the probe barrel at an offset angle from the longitudinal axis of the probe barrel. A typically cylindrical shaped probe tip extends partially out of the end of each probe barrel nose cone and is make of an electrically conductive material. A probe cable having an outer shielding conductor and a central signal conductor is connected to each of the probe barrels and the probe tips with the outer shielding conductor being connected to the probe barrel and the signal conductor being connected to the probe tip. An elastic compressible element engages each probe barrel and the probe unit housing allowing movement of the probe barrels into and out of the probe unit housing. The offset longitudinal axes of the probe nose cones and associated probe relative to the longitudinal axes of the probe barrels allows variable spacing of the probe tips.
The forces exerted on the probe barrel and probe nose cone assemblies are shown graphically in FIG. 1. The '670 patent shows the elastic compressible elements as compression springs following Hook's Law of F=K1 ΔX where K1 is the compression spring constant. FIG. 1 shows the forces applied to each of the probe nose cone and probe barrel assemblies during use, where “F” is the force applied to the probe tip of the probe nose cone and ΔX is the spring compression. Assuming that the elastic compressible elements are pre-loaded, there in an initial force on the assemblies as represented by the force F1. Downward force on the probe unit housing exerts an increasing force on the assemblies as represented by the sloping line K1. Continued downward force on the probe unit housing causes the elastic compressible elements are completely compress or the assemblies engage a fixed. Continued downward pressure on the probe unit housing transfers forces to the assemblies as represented by the vertical force line. The above described differential measurement probe is used for measuring signal from a device under test. As such, the differential measurement probe has passive input circuitry the lessens the need for EOS/ESD protection. Therefore, these probes do not ground the signal input to discharge electrostatic voltages on the device under test.
A further variable spacing differential measurement probe is the P7380 Differential Measurement Probe, manufactured and sold by Tektronix, Inc. Beaverton, Oreg. The P7380 probe has a probe body containing active circuitry and a probe tip member connected to the probe body by two coaxial cables. The probe tip member has differential probing contacts that mate with various probing tips mounted in a tip clip. One of the tip clips has rotatable probing tips that provides variable spacing for the probing tips. The probe body and the probe tip member may by positioned in a hand-held probing adapter housing that provides for easy hand-held probing by the P7380 as described in U.S. patent application Ser. No. 10/856,290, filed on May 27, 2004. The probe tip member is positioned in a cavity at the front of the hand-held probing adapter with a portion of the probe tip member extending past the end of the hand-held probing adapter. Within the cavity are compliant members formed of an elastomeric material that abut the side surfaces and rearward surfaces of the probe tip member. As the differential probing tips are brought into contact with a device under test, any non-planar variation between the probing tips and the device under test is taken up by the compliant members. The forces exerted on the probing tips of the probe tip member are shown graphically in FIG. 1. The compliant members are preferably formed of elastomeric material that are partially compressed by the probe tip member producing an initial pre-load condition as represented by the force F1. Downward pressure on the probe tip member exerts an increasing force on one or both of the probing tips as a result of the compressive characteristics of the elastomeric material as represented by force K1. Continued downward pressure on the probe tip member completely compresses the elastomeric material or the or the probe tip member engages a fixed stop. Continued downward pressure on the probe tip unit transfers forces to the probing tips as represented by the vertical force line. As with the previously describe variable spacing differential probe, the above described variable spacing differential measurement probe is used for measuring signal from a device under test. As such, the variable spacing differential measurement probe has passive input circuitry the lessens the need for EOS/ESD protection. Therefore, these probes do not ground the signal input to discharge electrostatic voltages on the device under test.
U.S. Pat. No. 6,734,689 describes a measurement probe providing signal control for an EOS/ESD protection control module. The measurement probe has a spring loaded coaxial probe assembly and a pressure sensor that work in combination to provide an activation signal to the control module. The spring loaded coaxial cable assembly and pressure sensor are disposed in a probe housing. The spring loaded coaxial probe assembly has a semi-rigid coaxial cable with one end forming a probing tip and the other end having a threaded connector. A flexible coaxial cable is connected to the threaded connector and to the control module. A compression spring is positioned over the semi-rigid coaxial cable with one end secured to the semi-rigid coaxial cable and the other end engaging the probe housing. The compression spring is pre-loaded to apply an initial force to the spring loaded coaxial probe assembly as shown graphically in FIG. 1. FIG. 1 shows the forces applied to the probing tip of the spring loaded coaxial probe assembly during use where “F” is the force applied to the probing tip, k1 is the spring constant, and ΔX is the displacement of the spring from its equilibrium position. The pre-loading of the compression spring generates an initial force F1 on the coaxial probe assembly. The pressure sensor has one electrical contact attached to the outer shielding conductor of the semi-rigid coaxial cable which is connected to electrical ground via the flexible coaxial cable. The other pressure sensor electrical contact is mounted to the probe housing. An electrical conductor electrically couples the pressure sensor to the control module.
The control modules provides a ground circuit path for the signal conductor of the measurement probe when the activation signal is absent. When the probing tip makes contact with the device under test, any static electricity on the DUT is coupled via the signal conductor to ground via the control module. As downward pressure is applied to probe housing, the coaxial probe assembly retracts into the probe body. The compression spring exerts increasing pressure on the coaxial probe assembly following Hook's Law of F=K1 ΔX where K1 is the spring constant. Continued downward pressure applied to the probe housing results in the pressure sensor contacts making contact. This results in the pressure sensor passing an activation signal which controls switching circuitry in the control module that removes a ground connection on the signal conductor of the measurement probe. Since the pressure sensor contacts are fixed to the semi-rigid coaxial cable and the probe housing, any continued downward pressure on the probe housing transfers the forces to the pressure sensor and the coaxial probe assembly as represented in FIG. 1 by the vertical force line. The excess forces on the pressure sensor and the coaxial probe assembly may result in damage to the pressure sensor or the coaxial probe assembly.
What is needed is a differential measurement probe having retractable double cushioned variable spacing probing tips and EOS/ESD protection capabilities. The variable tip spacing differential measurement probe needs to discharges static voltages on a device under test prior to the probing tips of the differential measurement probe being coupled to the signal channels of a sampling head. Further, the differential probe having retractable double cushioned variable spacing probing tips should provide an indication to a user that adequate pressure has been applies to the probe so as to prevent damage to the probe.